SVG renders should not paint when obscured
I'm not sure how we could ever implement this efficiently. But, ideally SVG elements would not paint when they are fully obscured (and even better, clip when they're partially obscured). The desire for this behavior is made clear when using something like: http://research.sun.com/projects/lively/index.xhtml which has opaque "windows" which obscure other animated content (yet the content still draws below it causing unnecessary repainting).